1. Field of the Invention
The present invention relates to a process controller and more particularly to a process controller which is suitable for control of a plant, such as a thermal power plant, in which a response gain of the process, a time constant, a dead time and the like vary in a non-linear manner in accordance with a load level, and in which the time constant and the dead time are large.
2. Description of the Related Art
With respect to prediction control there is known that described in an article "Proportional-Plus-Integral Prediction Control Method", Trans. IEEE of Japan, Vol. 105-C, No. 6, June, 1985, in which a control amount after n minutes is predicted and an operation amount is calculated on the basis of a deviation between that actual control amount and the desired amount after n minutes by the proportional-plus-integral prediction control.
As for the control which is considered impossible since a time constant and a dead time are large and the characteristics thereof change considerable and in a non-linear manner in accordance with a load, steam temperature control of a boiler of a thermal power plant is conventionally known.
For example, with respect to such control in which the main steam temperature of a boiler after n minutes is predicted using the Kalman filter and an operation amount is calculated by the proportional-plus-integral control on the basis of a deviation between that main steam temperature and the target value after n minutes, disclosure has been made in an article "Steam Temperature Prediction Control for Boiler Using the Kalman Filter", instrumentation, an extra number, 1983, pp. 113-115 and in an article "Steam Temperature Prediction Control for thermal Power Plant", IEEE Transaction on Power Apparatus and System, Vol. PAS-103, No. 9, September, 1984.
Further, as for such control in which the technology disclosed in the above article entitled "Steam Temperature Prediction Control for Boiler Using the kalman Filter" is applied, there is known an article "Improvements in Operation of Machine 2 of Niigata Port Thermal Power Station", Thermal and Nuclear Power Generation, Vol. 41, No. 1, Jan. 1990.
As for a method in which a model of a thermal power generation plant is used for correcting parameters of a control system, particularly in which a dynamic characteristic model of a thermal power generation plant is obtained by the autoregression method, using the resultant model, optimal values of PID control parameters are obtained by conventional technique. such as the critical sensitivity method, and the results are printed to be displayed, as described in JP-A-61-3202.
In addition thereto, with respect to a method in which the neuro-fuzzy is applied to a plant control, the following control methods are known:
As for the method in which the fuzzy inference is applied to a proportional-plus-integer 1 control system to realize an autotuning system, there is known an article "Autotuning System for PID Controller to which the Fuzzy Inference is Applied", Hitachi Technical Review, Vol. 71, No. 8, August, 1989.
As for the method in which when the standard starting schedule is corrected with respect to a starting equipment of a power generation plant, the simulation by the dynamic characteristic model of the plant is performed by means of the fuzzy inference, JP-A-63-945005, JP-A-63-94007, JP-A-6394008, and the U.S. Pat. No. 4,868,754 are known.
As for the method in which an identification neural network is made to learn the characteristics of the process, and by using a resultant model, the parameters of the neural network model for the optimization solution are determined thereby to determine the optimal operation amount, JP-A-2-161501 is known.
Hitherto, as the control method of the steam temperature of the boiler of the thermal power generation plant, the following methods are taken.
(1) As described in JP-B-64-10721 for the purpose of reducing an overshoot or an undershoot from a target value which is caused by the manipulation delay due to a large time constant and dead time of the process, an operation amount is maintained at the same value as in the current state, a control amount after one control period is predicted using an equation of state of the process with the current value of the control amount being treated as an initial value, the equation of state is repeatedly used with the predicted value being treated as an initial value, to thereby predict a deviation between a steam temperature of the future and a target value thereof, a proportional-plus-integral calculation result based on the deviation is added to a fuel operation command signal in the form of a correction signal, and the composite signal is treated as an operation amount signal to control the process.
(2) As described in JP-A-58-40612, a method is employed such that a control system is made to include therein the total system model of the process, and by using the control system, an optimal value of an operation amount is searched on the basis of the mathematical programming. In other words, the non-linearity is modeled in the form of table information, or is expressed in the form of a physical formula, to thereby obtain an optimal value of the operation amount using the non-linear planning which is typified by the complex method.
In the above-mentioned prior art, there arise the following problems